Multiple-input multiple-output (MIMO) transmission schemes are an advanced antenna technique to improve the spectral efficiency of a wireless communication system, thereby boosting the overall system capacity. The notation (M×N) is commonly used to represent MIMO configuration in terms of the number of transmit antennas (M) and receive antennas (N). In currently deployed systems, common MIMO configurations used for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (2×4), (4×4), (8×4). In addition, the 3rd-Generation Partnership Projection (3GPP) is discussing the possibility of extending the number of antennas at a base station up to as many as 64, thereby allowing additional configurations.
It is well known that MIMO systems can significantly increase the data carrying capacity of wireless systems. MIMO can be used for achieving diversity gain, spatial multiplexing gain and beamforming gain. For these reasons, MIMO is an integral part of 3rd and 4th generation wireless systems. In addition, massive MIMO systems are currently under investigation for fifth-generation (5G) wireless systems.
FIG. 1 illustrates multi-antenna transmission in LTE systems, the 4th-generation system specified by 3GPP. Antenna mapping, or precoding, in general, may be described as a mapping from the output of the data modulation to the different antennas ports. The input to the antenna mapping thus consists of the modulation symbols (QPSK, 16QAM, 64QAM, 256QAM) corresponding to one or two transport blocks of user data. To be more specific, there is generally one transport block per transmission-time interval (TTI), except when spatial multiplexing is used, in which case there may be two transport blocks per TTI. The output of the antenna mapping is a set of symbols for each antenna port. The symbols of each antenna port are subsequently applied to the OFDM modulator—that is, mapped to the basic OFDM time-frequency grid corresponding to that antenna port.
3GPP LTE provides several different variations on MIMO techniques, from beamforming to spatial multiplexing or single antenna schemes. A particular scheme is employed at any given time through selection of one of 10 Transmission Modes (TMs). These TMs are explained below.
Transmission mode 1: Single Transmit Antenna Mode. Support for this mode is mandatory for all terminals, and is used for base stations having only a single transmit antenna. This can also be used in cases where using more than 1 transmit (Tx) antenna is not feasible (e.g., in certain antenna sharing scenarios with other 2G/3G technologies).
Transmission mode 2: Open Loop Transmit Diversity Mode. In this mode, the same information is transmitted through multiple antennas, each with different coding/frequency resources. Alamouti codes are used as the Space Frequency Block Codes (SFBC) with two antennas. This transmission scheme is also a common fallback mode to single-layer transmission with dynamic rank adaptation in other transmission modes. TM2 uses Space Frequency Block Coding (SFBC) for 2TX and SFBC+Frequency Shift Time Diversity (FSTD) STX for 4TX.
Transmission mode 3: Open Loop Spatial Multiplexing with Cyclic Delay Diversity and Open Loop Transmit Diversity. This mode is also called open-loop single-user MIMO. As an open loop mode, this requires no Precoding Matrix Indicator (PMI), but only rank is adapted. Due to its simplicity, this is the widely deployed mode during the initial deployments of 3GPP LTE.
Transmission mode 4: Closed Loop Spatial Multiplexing (SU MIMO for rank 2 to 4). This has been the primary configuration for the majority of initial Release 8/9 deployments, used when the propagation channel supports transmission ranks from 2 to 4. TM4 multiplexes up to four layers onto up to 4 antennas. To allow the user equipment (UE) to estimate the channels needed to decode multiple streams, the eNodeB transmits Common Reference Signals (CRS) on prescribed Resource Elements. The UE replies with a PMI indicating which precoding is preferred, from the pre-defined codebook. This is used for Single User, SU-MIMO. When the UE is scheduled, a precoding matrix is selected and the UE is informed explicitly or implicitly which precoding matrix was used for the actual PDSCH transmission.
Transmission mode 5: Closed-Loop Multi-User MIMO for ranks 2 to 4. This mode is similar to TM4, but for the multi-user case, where multiple users are scheduled within the same resource block.
Transmission mode 6: Closed-Loop Rank-1 Precoding. This mode uses PMI feedback from the UE to select a preferred (one-layer) codebook entry (precoding vector) from the pre-defined rank 1 codebook. Since only rank 1 is used, beamforming gain is expected in this mode, but there is no spatial multiplexing gain.
Transmission mode 7: Single-Layer Beamforming. In this mode, both the data and demodulation Reference Signals (DMRS) are transmitted with the same UE-specific antenna precoder. With this approach, the UE does not distinguish the actual number of physical antennas used in the transmission and it does not know the actual precoding weights used as in the classical beamforming approach (TM6). TM7 is mainly used with TD-LTE, where the downlink channel state is well characterized by uplink measurements, due to reciprocity.
Transmission mode 8: Dual-layer beamforming. This mode was introduced in Release 9 of the 3GPP specifications for LTE. TM8 uses classical beamforming with UE-specific DMRSs, like TM7, but for dual layers. This permits the base station to weight two separate layers at the antennas, so that beamforming can be combined with spatial multiplexing for one or more UEs. The two layers can be targeted to one or two UEs.
Transmission mode 9: 8-layer MU-MIMO. TM9 was introduced in Release 10 of the 3GPP specifications. TM9 uses 2, 4, or 8 channel state information reference signals for measurements (CSI-RS) as well as 1 to 8 UE-specific DMRSs. Hence, it is a generalization of TM8 for up to 8-layer transmission. The introduction of the new CSI-RS enhances the CSI feedback. TM9 is suitable for MU-MIMO with dynamic switching from SU-MIMO. It is applicable to either to time-division duplexing (TDD) or frequency-division duplexing (FDD) systems, and support for TM9 is mandatory for terminals of Release 10 or later.
Transmission mode 10: An enhancement of TM9 where the resources used for interference measurements are further defined by the introduction of new CSI-IM resources. Support for TM10 is optional for terminals of Release 11 or later.
FIG. 2 shows a typical message sequence chart for downlink data transfer in LTE. From the pilot or reference signals, the UE computes channel estimates, and then computes the parameters needed for CSI reporting. The CSI report consists of, for example, channel quality indicator (CQI), precoding matrix index (PMI), and rank information (RI).
The CSI report is sent to the eNodeB (LTE terminology for the base station) via a feedback channel. The eNodeB scheduler uses this information in choosing the parameters for scheduling of this particular UE. The eNodeB sends the scheduling parameters to the UE in the downlink control channel called the Physical Downlink Control Channel (PDCCH). After that, actual data transfer takes place from eNodeB to the UE, via the Physical Downlink Control Channel (PDSCH). As discussed above, in some cases the UE uses CRS to obtain channel estimates for demodulating the PDSCH, while in others (e.g., TMs 7-9), the UE uses DMRS.
The several downlink reference signals mentioned above are predefined signals occupying specific resource elements within the downlink time-frequency grid. The LTE specification includes several types of downlink reference signals that are transmitted in different ways and used for different purposes by the receiving terminal:
Cell-specific reference signals: These reference signals are transmitted in every downlink subframe and in every resource block in the frequency domain, thus covering the entire cell bandwidth. The cell-specific reference signals can be used by the terminal for channel estimation for coherent demodulation of downlink physical channels, such as the PDCCH and PDSCH. CRSs are not used for demodulation of the PDSCH in the case of transmission modes 7, 8, or 9, however. CRSs can also be used by the terminal to acquire CSI. Finally, terminal measurements on CRSs are used as the basis for cell-selection and handover decisions.
Demodulation reference signals: These reference signals, sometimes referred to as UE-specific reference signals, are specifically intended to be used by terminals for channel estimation for PDSCH in the case of transmission modes 7, 8, 9 or 10. The label “UE-specific” relates to the fact that each demodulation reference signal is intended for channel estimation by a single terminal. That specific reference signal is then only transmitted within the resource blocks assigned for PDSCH transmission to that terminal.
CSI reference signals: These reference signals are specifically intended to be used by terminals to acquire CSI in the case when demodulation reference signals are used for channel estimation. CSI-RS have a significantly lower time/frequency density, thus implying less overhead, compared to the cell-specific reference signals.
In LTE, the downlink control channel (PDCCH) carries information about scheduling grants. Typically, this consists of information indicating a number of MIMO layers scheduled, transport block sizes, modulation for each codeword, parameters related to hybrid automatic repeat request (HARQ), and sub-band locations for the PDSCH. Note that with DMRS, there is no need to inform the UE of the selected precoding matrix, which reduces the number of bits that must be carried in the downlink control channel.
In Release 11 of the specifications for LTE, the evolved PDCCH (ePDCCH) was introduced. This alternative control channel, which is used to carry scheduling, e.g., downlink control information (DCI), uses time-frequency resources (resource elements) of the LTE signal that are normally allocated to the PDSCH, and can be dynamically precoded, using DMRS. However, the use of ePDCCHs has the restriction that the DMRS used for the ePDCCHs are common for all the ePDCCHs. This means that the pre-coding cannot be optimized for each of several UEs receiving an ePDCCH in a given TTI, since all the ePDCCHs in that subframe are using the same DMRS.
5G wireless systems currently under development are expected to support many antenna elements, enabling advanced pre-coding (beamforming), whereby transmission of data and control are both UE-specific (i.e., optimized for each UE). However, transmissions of data and control generally have different robustness requirements, as well as different requirements on the number of transmission layers needed to support their transmission. Further, the error rates which are suitable for the data and control are different. Accordingly, the modulation and coding of control and data may, as a general matter, be different. The problem with these different requirements has previously been solved by using different reference signals for demodulation of control and data, as was shown in the discussion of LTE above. In 5G systems, where a more flexible approach to the use of resources for data and control is desired, the current approaches are likely to be unsuitable.